The only way in which the apparently con tradictory results of experiment and observation can be reconciled, is by attending to a combina tion of vital and physical processes, that has been too much neglected in investigating the characters of living bodies ; that is to say, it must be recollected that " life," to borrow the philosophic expression of Dr. Arnott,* " is a superstructure on physics and chemistry," and that those phenomena which are essentially de pendent on the ordinary laws of matter are controlled and modified by the superior prin ciple of life. In the case of the cellular sub stance this remark is peculiarly applicable ; and from reflecting on all the facts relative to that tissue both in a state of health and disease, I have arrived at the conclusion that the phe nomena of its contractile force are the com bined results of one of the common proper ties of matter, viz., elasticity; and of a vital process, viz., nutrition. It is a well-known fact that the existence of elasticity in any inor ganic substance requires a particular state or arrangement of its particles, and that if the necessary condition be but partly fulfilled, or be entirely wanting, that property is only slightly displayed, or is totally absent. The same principle strictly applies to the living body; and in the cellular substance the required condition is, a definite proportion between the solid fibres and the interstitial fluids, which state is maintained by the agents of the circu lation and secretion, namely, the bloodvessels and lymphatics. Any thing which interferes with this proportion, either the excess of fluids, as in anasarca or plilegmonous erysipelas, or the diminution of the humours, as in old age and in many diseases, will impair or destroy the phenomena observable in the sound state of the cellular membrane, and will explain in the former case, the pitting which is seen on making pressure on the skin ; and in the latter, that flabbiness and wrinkling of the integument about the face and other parts of the body, so characteristic of those advanced in life or re duced by disease. We can in this manner understand how a class of phenomena may be dependent on a physical property, and yet be modified by the condition of the vital powers, so as to become impaired by disease, and destroyed by death.
The exhalation and absorption of which the cellular substance is the seat, have been sup posed by many high authorities to be effected by its elastic contractility ; but it is probable that these phenomena, although in part de pendent on that property, are principally pro duced by the power of imbibition, which, ac cording to the experiments of MM. Magendie and Fodera, exists in all the soft parts of the body.
Functions.—The offices accomplished by this substance in the economy seem to be, first, that of uniting together the various constituent parts of the body, and of keeping them in situ by its contractile force ; secondly, of facilitating their movements by means of its lubricating fluid, and thus preventing the injurious effects of friction and concussion ; and lastly, of fur nishing an appropriate structure for their recep tion. It has also been supposed that, being. a bad conductor of caloric, it will tend to pre serve the uniform temperature of the body.
Development.—The first trace of an organized substance observed in the embryo consists of a very soft and pulpy cellular tissue, which at this early period is loaded with fluid ; and being homogeneous in its nature, it presents neither fibres nor interstices, although it may be readily permeated by air or liquids, so as to produce small cells, and may likewise be drawn out into glutinous filaments. In proportion as
the several organs become developed, it acquires greater consistency, and is at the same time diminished in quantity. At the period of birth it is still, however, in a very soft and imperfect state, and only acquires its proper density by slow degrees ; in old age, being deprived of a large portion of its fluid, and perhaps otherwise deteriorated, it loses much of its elastic force ; and this circumstance, joined to its diminished bulk, is a principal cause of that loss of rotun dity so conspicuous in the bodies of aged persons, and of the flabbiness of' the several organs.
The power of reproduction is greater in this than in any other tissue, so that it is not only readily formed again within certain limits when it has been destroyed, but it even appears to supply the place of other and dissimilar struc tures which may have been lost by disease.
The cellular substance presents but few mo difications of importance when examined in the different classes of animals, except, indeed, tha,t it is generally believed to constitute the entire body in those species that are placed at the bottom of the scale. The Porifera afford.an exarnple of the simplest form of the cellular tex ture with which we are at present acquainted ; the body of these animals consists of a soft gelatinous substance composed of translucent globules, which, however, are not perceptibly joined together ; so that there is in this instance nothing of that fibrous structure, which is the great characteristic of the cellular membrane in the human body and in the higher orders of ani mals. In the semifluid and jelly-like body of the Polypifera and of some of the Acalephw, there is merely a pulpy substance, which, although it may exhibit a distinct digestive cavity, and even tubes communicating with this, yet no mus cular tissue has hitherto been discovered. In these animals, however, rapid movements are seen in the cilia; and the tentacula, when pre sent, together with the entire body, are capable of spontaneous motion ; it is evident, then, in these and other instances, that if, as is gene rally supposed, there be an absence of muscles, the cellular tissue must be endowed with a pro perty totally wanting in that substance as it exists in the higher animals. When it is con sidered how little is known respecting the real structure of the Infusoria, Zoophytes, &c., and when the numerous discoveries which have of late years been made in these and much higher animals, of parts whose existence was formerly doubted or denied, are recollected, we shall be inclined to think that there are special organs of motion provided ; for it would be in direct oppo sition to the simple but constant laws observed in the animal creation, were the organic tis sue, entitled the cellular, to acquire in the lower classes a power of contraction, which in thE higher it does not possess, and which property is the endowment of a totally distinct system of organs, namely, the muscles. Whichsoever of these opinions be correct, there is no doubt that in the least perfect animals, a soft and gelatinous matter, analogous to the cellular tissue, and loaded with fluids, greatly predominates. As we advance in the scale, it is found that organ ized substances of a diversified character are developed in the nidus afforded by this cellular texture, the proportion of which to the other structures becomes thus diminished.